RFR: 8279834: Alpine Linux fails to build when --with-source-date enabled

[Andrew Leonard]

The new reproducible build jar & jmod feature exposed a problem on Alpline linux whereby when --with-source-date is specified the IS_GNU_DATE setting was actually wrong. It was assuming the BusyBox "date" tool on Alpine linux was BSD compatible, but it is actually GNU syntax compatible.

This PR additionally checks for BusyBox when determining the date tool version compatibility.

Signed-off-by: Andrew Leonard <anleonar at redhat.com>

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Commit messages:
 - 8279834: Alpine Linux fails to build when --with-source-date enabled

Changes: https://git.openjdk.java.net/jdk/pull/7025/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7025&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279834
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7025.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7025/head:pull/7025

PR: https://git.openjdk.java.net/jdk/pull/7025